Process of concentrating nitric acid.



R. SOHLMAN & C. 0. LUNDHOLM. PROCESS 0F CONGENTRATING NITRIC ACID. APPLICATION FILED APR. 19, 1909.

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R. SOHLMAN- & c. 0. LUNDHOLM. PROGESS'OF GONGENTRATING NITRIC ACID.

APPLICATION FILED APR. 19, 1909.

1,009,197, Patented Nov. 21, 1911.

2 SHEETS-SHEET 2.

'rnn srAEs i RAGNAR SOHLMAN, OF BOFORS, SWEDEN, AND CARL 0L0}? LUNDHOLM, OF ARDEEB.

. SCOTLAND.

PROCESS OF CONGENTRA'IING NITRIC ACID.

Specification of Letters Patent.

Application filed April 19, 1909. Serial'No. 490,960.

' To all whom it may concern:

Be it known that we, RAGNAR SoHLMAN,

a subject of the King of Sweden, and resi-' dent of Bofors, Sweden, and CARL OLOF LUNDHOLM, a subject of the King of the United Kingdom of Great Britain and Ireland, and resident of Ardeer, Scotland,have invented new and useful Improvements in Processes of Concentrating Nitric Acid, of which the following is .a specification, reference being had to the accompanying drawing, which forms a part of this specification.

This invention relates to an improved process of concentrating nitric acid.

In the production of nitric acid by oxidizing the nitrogen of the air by the influence of the electric arc the acid heretofore has been obtained in a very diluted condition, generally containing about 25%, and at most to 40% of pure nitric acid, and has thereafter been utilized by being transformedinto nitrate, the solution having been neutralized by a suitable base, usually lime, whereupon the solution of nitrate has been evaporated to dryness.

The reason why the concentration of the acid in the process of production heretofore generally used cannot be driven over the limit. above stated, is that by reason of the percentage of oxids of 'nitrogen'of the air coming from the electric furnace being very small, an effective washing of the gas mixture with a weak acid and water is required for binding as perfectly as possible the comacid.

The greater part of the electric energy is transformed into heatin oxidizing the nitrogen of the-air under the influence of the electric arc, so that the'gases comin from the electric furnaces in most cases avea comparatively high temperature. The heat of said gases heretofore has been utilized for generating steam,-the gases having been :led from the furnaces through steam generators. .However, a great deal of said heat has been wasted, inasmuch as it has been neoessary to, further cool the gases to the usual temperature by means of cool ng water, in

.order to secure the absorption of thecombinations 'of nitrogen in the absorption system v Thepresent invention relates to a process by'which it is p'ossible'to produce, from the .aed' of. 25. t 4 r c d b re r nitrogen in the form of nitric methods, a concentrated acid of 90 to 97%, such as is generally used for industrial purposes, the said process rendering it possible to utilize in a rational manner the heat of hot gases, and being, theref dre, suitable for use in connection with the production of nitric acid by the electric oxidation of nitrogen of the air. The invention is hereinafter more particularly described with respect to such application thereof, and in the accompanying drawings, wherein like parts are similarly designated, we haveillustrated a concentration plant for carrying our invention into practice.

Figure 1 is a side elevation of the plant, partly in section, and Fig. 2 is a horizontal section on line IIII of Fig. 1. Fig. 3 shows diagrammatically a modification of a part of the plant. Fig. 4 is 'a sectional View on a larger scale, of the tower 14, 'Fig. l.

The concentration of the acid takes place in two steps, viz. (a) from 25 to 40% to about by direct contact with the furnace gases in suitable towers or tower systems, preferably based on the principle of counter-currents: (b) from about. 60% to 90%97% by distilling the nitric. acid from a mixture of the acid of 60% obtained according to (a) and sulfuric acid, in suitable. continuously working apparatus, heated externally by the furnace gases.

The cooling of the ases takesplace in three main steps, viz. 1) By leading them below or around suitable, continuously work.- ing apparatus for concentrating sulfuric acid, in which the dilute sulfuricacid ob-* tained in distilling concentrated nitric acid from its mixture with sulfuric acid '.(according to b above described) is again concen-. trated to its original strength." (2) 'Byleading the gases around distillation towers :or. similar apparatus, mentioned aboveunder b, in which the separation of the nitric acid from the mixture of sulfuric acid and nitricacid takes place. (3) By direct contact with weak nitric acid obtained according to a above, whereby the heat of. the gases ,is. utilized for vaporizing water from the dilute acid. No one of the above mentioned steps isv novel per se, each of them having beenpreviously used in different departments of f as set forth, a result heretofore not reached will be attained with respect to the effective -utilization of the heat, as well as with re- I forth, in transferring continuously about of the quantity of nitric acid, which according to well known and .generally used methods can be produced by using electric nitrifyingfurnaces, into strong acid of 95% to 97%, and of a very high degree of purity.

In carrying out the invention the concentration of sulfuric acid as mentioned above, lowers the temperature of the gases from about 700 C. to about EO-500 C., the distillation of the nitric acid lowers the temperature of the gases from 500-450 C. to 350300 (3., while the remaining heat down to 5075 C. is used for concentrating the weak nitric acid.

Referring to the drawings, 1 (Fig. 2) indicates an electric furnace in which the nitrogen of the air is oxidized. The hot gases coming from said furnace are led through the heat-insulated channel 2 to the boilers 3-4, which are surrounded by the gases. The said boilers are adapted for concentrating sulfuric acid; the dilute sulfuric acid (of about 78%) is supplied continuously through the pipe 5, a .part of the water distils oflt together with a small quantity of acid, through the pipes 6,7, which are connected to suitable "cooling 'ap paratus, 8, 9, and the concentrated sulfuric" acid flows off through the pipe-1O to a cooling apparatus, and from thence to the acidegg 11, from which it is forced into the re-. ceptacle 12 through pipe 13. The hot furnace gases further pass through a tower apparatus 14 surrounding a'pipe 15 .of acidproof pig-iron, and through the pipe 16 to the bottom of the tower 17, then' upward through said tower, through the pipe 18 to the bottom of the tower 19, upward through thelatter and through pipe 20, tower 21,

pipe 22, tower 23 and pipe 24 tov the exhauster 25. From .the latter the gases, which are now perfectly cooled, pass through the pipe 26 to other well-known absorption towers or'apparatus which may fully absorb the remaining percentage of oxids of nitrogen in the gases. The latter towers form no part of the present invention, and only one of them is, therefore, shown in the drawing. At the top of the tower 23 is the reservoir 28, supplied from the acid-egg 27 with weak nitric acid of the' strength -obtained in the above-mentioned.

absorption system, not shown, for instance, nitric acid of 25%. During the passage through the tower 23 the said acid is some-.

what heated by the current of hot gases.

passing through the tower in an opposite direction. The acid thereupon flows down into the acid-egg. 29 and is progressed through reservolr 30, tower 21, acid-egg 31, reservoir 32, 'tower 19," acid-egg 33, reservoir 34,,and 'tower.17,- and is successively heated and concentrated by a fractional distillation in said towers. In the last mentioned tower 17 "the acid which has already been somewhat concentrated, meets the hot gases supplied to the said tower through the channel 16. The tower Works in the manner of a column apparatus, the acid being caused to boil, whereby water is continuously distilled oif together with some nitric acid, while the acid flowing down hecomes more and more concentrated and its boiling point is raised. In this manner it would be theoretically possible to produce acid of about 70%. In practice, however, it has been found to be most suitable, when using acid of a strength that can advanta:

geously be obtained in an absorption systemthe tower into the acid-egg 35 and is forced therefrom up into the receptacle 36.

For further concentration the nitric acid of 60% is mixed in well known manner with v as pig-iron with a high percentage of silicon,

filled with pieces of silica or'the like. Thenitric acid distils off through the pipe 38 V and is condensed in suitablev cooling apparatus, represented in Fig. 1 by the carboys 39, 40. .The sulfuric acid, which retains the greater part of the water contained in the mixture of the acids, flows oflt through the pipe 5, and is concentrated in the boilers 4, 3, in the manner described above.

Instead of using-an electric source of heat,

a current of gas heated by other means, for

by such gases, may obviously be used. Consequently the present invention maybe used in otherdepartments ofchemical industry,

as well asfor producing nitric acid in the electric way, for instance, for utilizing instance, gases of combustion, or air heated Y the waste acid and weak nitric acid obtained in the production of explosives.

' of the processmay of course bemodified in many ways without'}departing-"from the rinciple of our invention. Thus, instead 1 of usm'gfour pre-concentration towers 17,

19, 21, 23 in series, as shown in Fig. 1, the towers may be arranged in two parallel rows of two towers each, and the acid may be conveyed in zig-zag'from the one tower to the other, as is shown diagrammatically in Fig.

'3. The towers, 41, 42, 43, 44, here consti- 'from 44 by the gas escaping from the tower.

The acid thus pro-concentrated and preheated, is conveyed to tower 43, Where a further concentration takes place, say up to then to 42, Where the concentration is brought to, say 50%, and finally to 41, where the strength of the issuing acid is increased up to 5860%. This manner of operating has the advantage that a larger percentage of the weak acid fed to the last tower is recovered from the tower 41 as concentrated acid. A

Instead of using the apparatus described, for the purpose of distilling off strong nitric acid from its mixture with sulfuric acid, and for concentrating again the weak sulfuric acid thus obtalned, it is, of course, possible to use various other well known apparatus, working either continuously or intermittently. Thus, both these operations may be successively carried out by using a system of several pans of the same type as the pans 3, 4, shown-in Fig. 1, arranged in a series of, for instance, four. Or the concentration of surfuric acid may be effected in a tower apparatus of any well known type, heated by a part of the furnace gases passing through said tower.

We claim The process of concentrating weak nitric acid, which consists in making a preliminary concentration of nitric acid by direct contact with a current of hot gases, mixing the nitric acid thus concentrated with concentrated sulfuric acid, and distilling the nitric acid from the mixture and then concentrating the sulfuric acid for re-use in the process, one and the same current of hot gases being successively utilized .for concentrating the dilute sulfuric acid, for dis tilling strong nitric acid from the mixture of nitric and sulfuric acids, and for preconcentrating the weak nitric acid by direct contact with the hot gases, substantially as and for the purpose set forth.

In testimony that we claim the foregoing as our invention, we have signed our names in presence of two subscribing witnesses.

RAGNAR SOHLMAN. CARL OLOF LUNDHOLM.

Witnesses:

JOHN DELMAR, EWALD DELMAR. 

